At 1815 your GPS position is LAT 41°09.2'N, LONG 72°36.9'W. You are steering course 285° per standard magnetic compass at a speed of 16 knots. At 1909 you fix your position by plotting several compass bearings on nearby known fixed objects. These result in a position of LAT 41°08.5'N, LONG 72°53.7'W. What were your set and drift?
• Dead reckoning (DR) track versus fix position on a plotting sheet • How to compute a current vector (set and drift) as the difference between DR position and fix • Converting time difference and distance between DR and fix into speed in knots
• First, from 1815 to 1909, what is the elapsed time in minutes, and how many hours is that? • If you were steering 285° per standard magnetic compass at 16 knots, where would your DR position be at 1909 (before considering current)? • On the plot, what is the direction (true bearing) and distance from the DR position to the 1909 fix, and how does that vector translate into set (direction of current) and drift (speed of current)?
• Be sure you are measuring the vector from DR position TO the fix, not the other way around • Convert minutes of latitude to nautical miles carefully; 1 minute of latitude equals 1 NM • When computing drift, use ( \text{Speed (knots)} = \frac{\text{Distance (NM)}}{\text{Time (hours)}} ) and verify your time in hours, not minutes
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